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EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON
EXPERIMENTAL CONVERSION OF
DIOCTAHEDRAL SMECTITE TO BERTHIERINE, CHLORITE, VERMICULITE, OR SAPONITE
Régine Mosser-Ruck, Michel Cathelineau, D. Guillaume, Delphine Charpentier, D. Rousset, Odile Barres, Nicolas Michau
To cite this version:
Régine Mosser-Ruck, Michel Cathelineau, D. Guillaume, Delphine Charpentier, D. Rousset, et al..
EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON EX-
PERIMENTAL CONVERSION OF DIOCTAHEDRAL SMECTITE TO BERTHIERINE, CHLO-
RITE, VERMICULITE, OR SAPONITE. Clays and Clay Minerals, Clay Minerals Society, 2010, 58
(2), pp.280-291. �10.1346/CCMN.2010.0580212�. �hal-00566986�
EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON EXPERIMENTAL CONVERSION OF DIOCTAHEDRAL SMECTITE TO
BERTHIERINE, CHLORITE, VERMICULITE, OR SAPONITE
RE G I N E MO S S E R- RU C K1 ,* , MI C H E LCA T H E L I N E A U1, DA M I E NGU I L L A U M E2, DE L P H I N E CH A R P E N T I E R3, DA V YRO U S S E T1, OD I L E BA R R E S4, A N DNI C O L A SMI C H A U5
1G2R, Nancy-Universite´, CNRS, CREGU, Boulevard des Aiguillettes, B.P. 239, F-54506 Vandoeuvre-le`s-Nancy, France
2LMTG, UMR 5563 CNRS-UPS-IRD, Observatoire Midi-Pyre´ne´es, 14avenue Edouard Belin, 31400 Toulouse, France
3CNRS-Universite´ de Franche-Comte´/UMR 6249 Chrono-environnement, 16 route de Gray, 25065 Besanc¸on, France
4Laboratoire Environnement et Mine´ralurgie, CNRS UMR7569, 15 Avenue du Charmois, BP40, 54501 Vandoeuvre-le`s-Nancy, France
5ANDRA, Direction Scientifique/Service Mate´riaux, Parc de la Croix Blanche, 1/7 rue Jean Monnet, 92298 Chaˆtenay-Malabry, France
Abstract—In deep geological repositories for high-level nuclear wastes, interactions between steel canisters and clay-rich materials may lead to mineralogical transformations with a loss of the confining properties of the clays. Experiments simulating the conversion of smectite to Fe-rich clay phases in contact with Fe metal have been carried out to evaluate such a possibility by taking into account the effects of a series of critical parameters, including temperature, pH, and Fe/clay (Fe/C) and liquid/clay (L/C) ratios.
The mineralogical and chemical transformations observed in these experiments have been compared with data from the literature, and subsequently used to propose a conceptual model for the main mineralogical transformations which can be expected in clay formations surrounding high-level nuclear waste repositories. In the presence of Fe metal and under low oxygen fugacity (<10 40) the main mineralogical sequences are as follows:
(1) up to 150ºC, under neutral pH, and L/C > 5: dioctahedral smectite (di-sm)?7 A˚ Fe-rich phase (berthierine, odinite cronstedtite) for large Fe/C ratios (>0.5), or di-sm ? Fe-rich di-sm + Fe-rich trioctahedral smectite (tri-sm) for small Fe/C ratios (0.1);
(2) up to 150ºC, under alkaline pH (10 12), and L/C > 5: di-sm?Fe di-sm (Npalygorskite) for a small Fe/C ratio (0.1);
(3) at 300ºC, Fe/C = 0.1, and L/C > 5: di-sm?Fe-rich saponite?trioctahedral chlorite + feldspar + zeolite (near-neutral pH); di-sm?Fe-rich vermiculite + mordenite (pH 10 12).
Low temperatures (<150ºC) and large L/C and Fe/C ratios seem to favor the crystallization of the serpentine group minerals instead of Fe-rich trioctahedral smectites or chlorites, the latter being favored by higher temperatures. The role of L/C and Fe/C ratios and the competition between them at different temperatures is a crucial point in understanding the transformation of smectite in contact with Fe metal.
Key Words—Berthierine, Chlorite, Fe Metal, Nuclear Waste, Saponite, Smectite, Vermiculite.
INTRODUCTION
The interaction between smectites and Fe metal has been investigated in recent years in response to queries by nuclear-waste management agencies about the long- term behavior of engineered barriers after closure of the repository. Previous works, dealing with Fe-clay inter- action, report the formation of distinct types of clays (serpentine-like phases, dioctahedral or trioctahedral Fe-rich smectites, chlorite) and distinct mineral sequences, and sometimes appear to be contradictory.
The contradiction has encouraged this comparison of previous experimental conditions and results with new data. In close contact with the waste packages (‘near-
field system’) and in reduced conditions, water is unstable, Fe is released, and H2is produced. The main parameters to be considered in such a system are shown in Figure 1 and a summary of the experimental conditions and experimental results described in the literature is presented in Table 1.
Experimental temperatures
Depending on countries and concepts, the initial temperature of the overpack in the repository will vary between 80ºC and 100ºC. The main problem which arises when conducting experiments on smectites at such temperatures is the difficulty in reaching equilibrium, especially when the L/C ratio is small. In engineered clay-barrier conditions, L/C is ~0.2N0.1, and it depends on the density of the bentonite. To compensate for kinetic effects, one possibility is to increase the temperature by conducting the experiments at greater
* E-mail address of corresponding author:
[email protected] DOI: 10.1346/CCMN.2010.0580212
Clays and Clay Minerals, Vol. 58, No. 2, 280–291, 2010.
temperatures, up to 250 300ºC (Guillaume, 2002;
Guillaume et al., 2003, 2004; Wilson et al., 2005, 2006; Charpentieret al., 2006). Another possibility is to increase the reactive surface by increasing the dispersion of particles and the amount of water.
Changes in redox conditions and Fe concentration in the fluids
After closure of the repository, the engineered barrier will be submitted to increasingly reducing conditions (1, 2, and 3 in Figure 1). The scenario includes (1) the consumption of residual oxygen introduced in the pores during the early steps through the oxidation of the Fe2+
phases such as pyrite; and (2) the production of H2due to the instability of water in the presence of Fe metal (Garrels and Christ, 1965). The Fe metal water inter- action results in a drastic reduction infO2 to the water instability boundary,e.g. within the field of magnetite + H2, and close to the Fe metal + H2field. At anfO2fixed by the boundary Fe metal/magnetite or less than thefO2 determined by the equilibrium:
H2O?H2+ W O2
the Fe metal is transformed into magnetite following the reaction:
3 Fe0+ 2 O2?Fe3O4
thus yielding the mass balance reaction:
4 H2O + 3 Fe0?Fe3O4+ 4 H2
Changes in pH and in the ionic strength
The penetration of alkaline fluids issued from the concrete-water interaction in other engineered parts of
the repository must result in an increase in pH of the interstitial fluid within the clay barrier. Alkaline fluids are generally progressively neutralized by the interaction with clays, as shown by experiments (trend a in Figure 1). The pH decreases from values of 12 or 10 to ~8.5 (at 80ºC for instance), when no alkaline reserve is introduced, e.g. when the alkalinity of the incoming fluid is only considered as a starting experimental feature and is not maintained (Charpentier et al., 2006). The dissolution of Fe metal may produce an increase in pH (trend b in Figure 1) if no other mineral reaction is involved:
Fe0+ 2 H2O?H2+ 2 OH + Fe2+
When an allochtonous source of fluids is considered, e.g. alkaline fluids from the concrete parts of the repository, the ionic strength of the incoming fluids in contact with the steel containers will increase. In such a case, the increasing activity of calcium stabilizes other kinds of minerals such as zeolites (Charpentier et al., 2006).
The reported experiments (Table 1) were carried out in autoclaves in the presence of air, e.g. with an O2
reserve (Lantenois, 2003; Lantenois et al., 2005;
Perronnet, 2004; Perronnet et al., 2008), under argon atmosphere (Guillaumeet al., 2003, 2004), or under N2
atmosphere (Wilson et al., 2005). Various amounts of Fe, as powders of metal (Fe0) and/or oxides (Fe2O3, Fe3O4), were added to the experimental media leading to different Fe/C ratios, from 0.1 to 2, according to the authors. Liquid/clay ratios also vary. Experiments were conducted at L/C values of 10 (Guillaume et al., 2004 ; Charpentier et al., 2006), 16.7 (Perronnet, 2004;
Perronnet et al., 2008; Lantenois et al., 2005), and Figure 1. Main parameters taken into account in the experimental modeling of the Fe clay interactions: (a) pH reduction due to alkaline fluid bentonite interaction; (b) pH increase due to Fe dissolution; (c) temperature-pH field chosen for new data in this study: (1) excavation under atmosphericfO2; (2)fO2reduction while O2stock lasts; and (3) instability of water.
Table1.Listofpublishedresultsasafunctionofdifferentphysicalchemicalexperimentalparameters. —————————————————————————Experiment—————————————————————————Mainnewly- MixtureLiquid/claymass ratio(L/C)Iron/clay massratio (I/C) Startingredox conditionsStartingpH ofexperimental solutions
TimeTemperatureformedclayphasesBy-productsAuthors MX80+Fe+magnetite10 (Na,Ca)-Cl0.1
Argon atmosphere Fe/magnetite (550mV measured)
~7112 months 80ºCand 150ºCFe-richdi-smNotdeterminedGuillaume(2002), Guillaumeetal.(2004) 300ºCFe-richtri-sm (saponitelike)+ Fe2+-chlorite
Quartz,feldspar, zeolite(erionitelike)Guillaumeetal.(2003) MX80+Fe+magnetite10 (alkalinesolution: NaCl+Ca(OH)2)0.1Argon atmosphere Fe/magnetite12.319 months
80ºCand 150ºCdi-smectiteTransitory palygorskiteCharpentieretal. (2006) 300ºCFe2+-rich vermiculiteMordeniteCharpentieretal. (2006) Montmorillonite (SWy2)/Beidellite (SBld-1)/Nontronite (Garfield)+Fe (Nodestabilization oftrioctahedral smectites)
16.7 (purewateror springwater)1to2Atmospheric orargon atmosphere7to1145days80ºC7A˚phases(gels): Odinite–cronstedtite seriesMagnetiteLantenoisetal.(2005) FoCa7orCa- smectite(Prassa)or Ca-nontronite+Fe16.7 (Evian) orNaCl 15mmolL1
0.13to0.33Atmospheric7.2
112 months 80ºC 7A˚phases:berthierine, odinite,cronstedtite (Fe/C=0.130.3and time=13months)MagnetitePerronnet(2004) Perronnetetal.(2008)Tournemire+FeNotobserved MX80+Fe3 months
Notobserved PurifiedMX80+Fe7A˚phases Na-montmorillonite+ magnetite+Fe+ calcite
66.7 (NaClsolution)0.57;0.68; 0.78
<hematite- magnetite N2atmosphere9.73to4.1593,114 days250ºCFe2+richsmectite (saponite-like) MagnetiteWilsonetal.(2005, 2006) Na-montmorillonite+ Fe
66.7 (FeCl2)~0.6>hematite- magnetite4.24to4.7190,92 days 80ºC,150ºC, 250ºC 7A˚phases(berthierine onlyat250ºC)
66.7 (Wilsonet al., 2005, 2006). Experiments carried out at high temperature to compensate for kinetic effects yielded to the formation of Fe-saponite and chlorite after dioctahedral Na-Ca smectite at 300ºC (Guillaumeet al., 2003) or berthierine-like minerals at 250ºC (Wilson et al., 2005, 2006). At lower temperatures (<150ºC), dioctahedral smectite was sometimes preserved (Guillaume, 2002; Guillaume et al., 2004; Charpentier et al., 2006). On the other hand, Lantenoiset al.(2005), Perronnet (2004), and Perronnet et al. (2008) reported the formation of a 7 A˚ phase (berthierine or a mineral of the odinite cronstedtite greenalite series) for experi- ments carried out at 80ºC. An overall consideration of the experimental conditions and experimental results compiled from the available data of the literature (Table 1) shows that the mineralogical assemblages obtained after experiment are probably very dependent on parameters such as the Fe/C and the L/C ratios. The purpose of the present study was to evaluate the role of these parameters by comparing published data (Table 1) to results obtained from a new set of experiments carried out at 80ºC, 150ºC, and 300ºC with different Fe/C ratios (0.1 and 0.5) and L/C ratios (5 and 10). The effects of time and temperature, as well as the impact of an increased availability of the Fe metal, were investigated.
Special attention was paid to the impact of the distance to the Fe plate on the nature of the run products.
MATERIALS, METHODS, AND ANALYTICAL TECHNIQUES
Batch experiments were conducted at 80, 150, and 300ºC in dilute chlorine solutions (NaCl, CaCl2) having ionic strength and major cation compositions close to the natural waters present in sediments and similar to that of experiments described by Guillaume (2002) and Guillaume et al. (2003, 2004). The starting product was a Wyoming bentonite (MX80) consisting of
>85 wt.% of smectite. The smectite is a montmorillonite and the half structural formula (<2mm fraction of the bentonite) given by Guillaumeet al.(2003) is:
(Si3.98Al0.02)(Al1.55Mg0.28FeII0.08FeIII0.09)O10(OH)2Na0.18Ca0.10
Bentonite was placed in contact with Fe oxides (magnetite) and Fe metal in autoclaves over periods ranging between 3 and 12 months. The L/C mass ratios were 5 or 10 (rather less than those used in previously published works, see Table 1), and Fe (powder of metal Fe + magnetite)/clay ratios were 0.5 or 0.1. A plate of metallic Fe (~1 cm2) was added in all experiments. The starting products were introduced into the autoclave under Ar atmosphere. Details of the experimental procedures have been published elsewhere (Guillaume et al., 2003, 2004; Charpentier et al., 2006). The mineralogical and chemical evolutions of the clays w e r e s t u d i e d b y X - r a y D i f f r a c t i o n ( X R D ) , Transmission Electron Microscopy (TEM) using High
Resolution and Electron Dispersive Spectroscopy (HR- and EDS-TEM), and micro-Fourier Transform Infrared (mFTIR) spectroscopy.
Analysis of particles was coupled with HRTEM images so that particle edges could be observed. X-ray diffraction, carried out on air-dried and glycolated preparations of samples, was used to investigate the swelling feature of the clays and to discriminate the main run products (saponite, vermiculite, and chlorite or berthierine). Serpentine group minerals and Fe-rich chlorite were extremely difficult to distinguish by XRD (because of the similarities of their chemistry and structure HRTEM was used to help in the identifica- tion; and because of the small amount of newly formed phases in comparison to the unmodified starting product).
Due to the number of run samples, crystal-chemical trends are presented as they, in particular, show clearly the increasing incorporation of Fe in newly formed phases, the permanence or otherwise of exchangeable cations typical of smectite-type or vermiculite layers, and the changes in the occupancy of the tetrahedral site by Si which was the best indicator of the smectite to non- swelling (chlorite or berthierine) conversion.
The FTIR spectra were obtained in the mid-IR region (4000 600 cm 1) by micro-Fourier Transform Infrared (mFTIR) spectroscopy using a Bruker IFS 55 spectro- meter associated with a Bruker A590 microscope. All measurements were conducted in transmission mode on samples deposited on ZnSe slides. The diameter of the beam was 60mm and the spectral resolution was 2 cm 1. The recording time was ~1 min. Each spectrum was processed using theOPUSprogram (#Bruker).
RESULTS Effect of Fe/clay ratio
80ºC experiments.At 80ºC for a L/C ratio of 10 and an Fe/C ratio of 0.1, run products were characterized by slight decreases in Si content and interlayer charge (Na + K + 2Ca) together with a slight increase in Fe content (Figure 2). The duration of experiment had no effect on the reaction and a possible equilibrium was obtained after 3 months. In contact with the Fe plate (large Fe/C ratio) and for a short duration (3 months), clay particles were richer in Fe (filled triangles in Figure 2) than the bulk run product.
A more significant evolution was observed for a greater Fe/C ratio (0.5) and for a smaller L/C ratio (5) (gray triangles in Figure 2): these experimental condi- tions favored the crystallization of an Fe-rich and Si- and Ca-poor phase. The duration of this experiment was important (12 months) but equilibrium was quickly obtained for shorter durations. The XRD patterns showed that the fine fraction (<4mm) of this last run product (Figure 3) was mostly composed of a swelling clay-like smectite. Additional saturations of this fraction
with Li and Mg before XRD analysis confirmed that a significant proportion of the run product was fully expandable. The run product could be thus a mixture of a smectite and an Fe-rich 14A˚ chlorite (chamosite type) or
a 7 A˚ clay (berthierine type) since the global interlayer charge of the particles decreases. The disagreement between XRD and EDS-TEM results could be explained by a mechanical enrichment in smectite particles during Figure 2. Comparison of the crystal-chemistry of the clay phase for experiments carried out at 80ºC with a L/C ratio of 10 or 5, and an Fe/C ratio of 0.1 or 0.5. The durations were 3 months (open triangles), 6 months (open diamonds), 9 months (open squares), or 12 months (gray triangles). Filled triangles correspond to particles sampled on the Fe plate. The circle corresponds to the range of values of the starting MX80 bentonite Sivs.Na + K + 2Ca; Fe (total Fe converted to Fe3+)vs.Mg; Cavs.Na; and Fe/(Fe + Mg)vs.Si diagrams plotted from EDS-TEM analyses and structural formulae calculated on the basis of 11 oxygens (diagrams after Graubyet al., 1993).
Figure 3. XRD patterns recorded from the fine fraction (<4mm) of the run product obtained in the 12-month experiment carried out at 80ºC in the presence of a large Fe/C ratio (0.5) and L/C ratio of 5: (a) air-dried sample (AD); (b) glycolated sample (EG: solvation with ethylene glycol vapor at room temperature over 24h); and (c) heated sample (H: 4h at 550ºC). The index of reflections corresponds todhklin A˚ . The XRD data were collected using a D8 Bruker diffractometer with CoKa1radiation (l= 1.7902 A˚ ). The general operating conditions were 35 kV accelerating voltage, 45 mA intensity, step-scanning at 0.035º2yintervals, 3 s counting time, 3 40º2yfor oriented powder.
284Mosser-Ruck et al. Clays and Clay Minerals
the separation of the <4mm fraction for X-ray analysis.
A HRTEM image of the bulk of this run product shows both numerous long flakes typical of smectite (10 A˚ interlayer) and several shorter and thicker particles with a 7 A˚ interlayer (Figure 4) and confirmed the coexistence of at least two types of clay in the run sample. The 7 A˚ Fe-rich phase was, therefore, mostly found for experiments with a large Fe/C ratio. The 7 A˚ phase had a composition close to that of berthierine (gray triangles in Figure 2) and its abundance decreased to undetectable amounts in experiments carried out with smaller Fe/C ratios. Clay particles of this run product were also sampled at the contact with the Fe plate and analyzed by FTIR spectroscopy. The IR spectrum (Figure 5) was compared to that of the starting
Na-montmorillonite (MX80). In the spectra for both the starting montmorillonite and the run product, bands o b s e r v e d b e t w e e n 3 8 0 0 3 1 0 0 c m 1 a n d 1700 1500 cm 1were assigned to OH-stretching vibra- tions from clay minerals and water and to water-bending vibrations, respectively. The spectrum of the run product showed several new bands at 1410, 1385, 1356, 712, and 667 cm 1. By comparison with literat ure data (Saumagne and Josien, 1962; Toppani et al., 2005), these bands were assigned to stretching (between 1410 and 1356 cm 1) and bending modes (712 and 667 cm 1) of the CO32
group. Otherwise, the bending band fitted at 1585 cm 1 was assigned to strongly bound water. This kind of FTIR spectrum was very similar to that obtained by Toppani et al. (2005) for complex hydrated carbonates. The starting MX80 bentonite containing
~2% carbonates would be the source of CO2 in the experimental system, allowing the formation of bicar- bonates at the contact with the Fe plate.
300ºC experiments. The impact of the Fe/C ratio on the transformation of montmorillonite at three different experiment durations (3, 6, and 9 months) was studied (Figure 6). Experiments with a small Fe/C ratio (0.1, diamonds in Figure 6) and with a large Fe/C ratio of 0.5 (squares) were compared. The EDS-TEM analyses showed that the increase in Fe/C ratio enhanced the chemical evolution. The decrease in Si content was more significant (Si <3 for a half formula based on 11 oxygens) in run products obtained in experiments with an Fe/C ratio of 0.5 for all durations and in the 9 month experiment only when the Fe/C ratio was 0.1. A long experiment (9 months) and a small Fe/C ratio had the same impact on the tetrahedral occupancy as a short experiment (3 months) and a large Fe/C ratio. Different Fe/C ratios also yielded distinct octahedral occupancies Figure 4. HRTEM analysis of the run product obtained from the
12-month experiment carried out at 80ºC in the presence of a large Fe/C ratio (0.5) and L/C ratio of 5.
Figure 5. IR spectra in transmission mode of the starting MX montmorillonite and the run product sampled at the contact of the Fe plate in 80ºC, 12 months, L/C = 5, Fe/C = 0.5 experiment.
of the run clays. In the small Fe/C-ratio (0.1) experi- ments, both Fe and Mg enrichments were observed in run products (arrows in Figure 6) whereas a large Fe/C ratio in the experimental medium led to a significant Fe enrichment of clays only. Interlayer occupancy of the run products was generally depleted both in Ca and Na for all run products. The new data obtained at 300ºC from greater Fe/C-ratio experiments were in good agreement with results obtained by Guillaume et al.
(2003) and Charpentieret al.(2006) who demonstrated that the chemical evolution of the montmorillonite in MX80 bentonite depends on the distance to the Fe plate and consequently on the availability of Fe in the medium. Iron enrichment in particles sampled near and at the contact with the Fe plate (where the Fe/C ratio was large) was always far more significant.
Effect of temperature
One of the most important parameters controlling the transformation of smectite is temperature. Its effect was also studied in the presence of a metallic Fe plate and under an Fe/C ratio of 0.5 and a L/C ratio of 5. Three long duration experiments (12 months) were conducted at 80ºC, 150ºC, and 300ºC. They showed that the increase in temperature both enhanced Si depletion and Fe enrichment of the run clays (Figure 7). The interlayer charges were different in newly formed clays yielding to t h e f o r m a t i o n o f F e - r i c h s a p o n i t e a t 3 0 0 º C (Na + K + 2Ca ~ 0.3 to 0.4) and the crystallization of a 7 A˚ phase mixed with starting smectite at a temperature of <150ºC (Na + K + 2Ca <0.2). In the greater Fe/C-ratio (0.5) experiments, no zeolite by- products were observed, regardless of the temperature.
DISCUSSION AND CONCLUSIONS Mineralogical transformations and sequences
From literature data (Table 1) and results presented in the present paper, at 80ºC and 150ºC, in the presence of Fe (powder and plate) and Fe oxides (magnetite), and for specific experimental conditions (large L/C ratio and Fe/C <0.5), montmorillonite evidently remains the pre- dominant clay mineral in run products. Under any pH (neutral or alkaline), similar weak enrichment of Fe and depletion of Si were observed in the clays. At 300ºC, vermiculite was predominant, associated with Fe-rich trioctahedral smectite (saponite-type) in alkaline condi- tions (Charpentieret al., 2006) whereas Fe-rich chlorites associated with Fe-rich trioctahedral smectite (saponite- type) were identified in neutral conditions (Guillaumeet al., 2003, 2004). The experimental crystallization of Mg-vermiculite after smectite in alkaline solutions was also mentioned by Chermak (1992). In the study of Charpentier et al. (2006), the formation of Fe-rich vermiculites seemed to be favored by the initially alkaline conditions and the presence of Fe metal, all other conditions being equal in comparison with the 300ºC experiments reported by Guillaumeet al.(2003).
In all cases, chemical zoning was observed from the very near contact with the Fe plate toward the bulk clay, usually a decrease in Fe in the run products sometimes in favor of Mg (at 300ºC).
The occurrence of quartz, feldspars, magnetite, and zeolites as reaction by-products in all experiments is consistent both with thermodynamic modeling predic- tions (Cathelineau et al., 2001; Kluska et al., 2002;
Monteset al., 2005) and data from other experiments on bentonites (Table 1). The simplified conversion of a Figure 6. Comparison of the crystal chemistry of the clay phases for 3-, 6-, and 9-month experiments carried out at 300ºC with a L/C ratio of 10, and different Fe/C ratios (0.1 or 0.5). Sivs.Na + K + 2Ca; Fe (total Fe converted to Fe3+)vs.Mg; Cavs.Na; and Fe/
(Fe + Mg)vs.Si diagrams are plotted from EDS-TEM analyses and structural formulae calculated on the basis of 11 oxygens (diagrams after Graubyet al., 1993).
286 Mosser-Ruck et al. Clays and Clay Minerals
montmorillonite to a chlorite of chamosite-type (Si3Al)(Al)(Fe2+,Mg)5O10(OH)8 (or berthierine which has nearly the same chemical formulae based on 5 oxygens) yields to the expulsion of silica, which crystallizes either as quartz or other silicates (feldspars, zeolites) depending on the availability of cations in solution. For the material studied, it can be expressed as follows:
2 (Si3.98Al0.02)(Al1.55Mg0.28FeII0.08FeIII0.09)O10(OH)2Na0.18Ca0.10+ Montmorillonite MX80 (Sm)
7.039 Fe2++10.76 H2O + 0.18 e ? 1.57 (Si3Al)(Al )(FeII4.7Mg0.3)O10(OH)8+
Chamosite or 3.14berthierine (chl)
0.2 Ca2++ 0.36 Na++ 3.25 SiO2 (aq.)+
0.089 Mg2++ 12.96 H+ (1)
As the dissolution of Fe metal is expressed as:
Fe0+ 2 H2O?H2+ 2 OH + Fe2+ (2) and reduction of Fe3+to Fe2+is expressed as:
Fe3++ WH2?Fe2++ H+ (3)
the combination of reactions 1, 2, and 3 results in:
7.039 Fe0+ 11.698 H2O + 2 Sm? 1.57 Chl + 0.2 Ca2++ 0.36 Na++
3.25 SiO2 (aq.)+ 0.089 Mg2+ + 6.94 9 H2+ 0.938 OH
The production of H+ in reaction 1 partly compen- sates the production of OH due to the dissolution of Fe (reaction 2), thus explaining the near-neutral pH of most experimental solutions (measured after quenching). The availability of silica and cations in solution led to the formation of feldspars or zeolites. The presence of bicarbonates or carbonates at the contact of the Fe plate in one experiment is analogous to corrosion products of the steel canister reported in the literature (e.g.Schlegel et al., 2008) and the results of geochemical modeling of Fe-clay interactions (Bildsteinet al., 2006; Savageet al., 2010). Discrete calcite originally present in the bentonite probably dissolved at the beginning of the experiment.
The Ca2+ions released can enter the interlayer space of smectite and HCO3 can combine with other cations as Fe2+ at the contact of the Fe plate to form hydroxy- carbonate (or carbonate) species. Neutral to high pH values at the contact with the Fe plate (as shown by the presence of magnetite in some experiments) and a small PCO2 could favor the formation of hydroxycarbonates.
Finally, the mineralogical sequences depend on the availability of Fe in solution and on the starting pH of the solution. Under neutral pH conditions, the sequence of newly formed mineral assemblages is thus as follows:
(1) dioctahedral smectite (di-Sm) > Fe-rich di-Sm for small Fe/C ratios and small L/C ratios, or berthierine- like mineral if the Fe/C ratio is >0.5 at 80ºC or 150ºC;
(2) di-Sm > Fe di-Sm > Fe-saponite > trioctahedral chlorite + feldpars + zeolite, at 300ºC.
Experiments carried out with a starting pH of 12 led to the following mineral sequences:
(1) di-Sm > Fe di-Sm + palygorskite at 80º and 150ºC, (2) di-Sm > vermiculite (N Fe saponite) + mordenite at 300ºC
The possible mineral zonings are, therefore, distinct when the fluids come from the geological barrier or from concrete.
Comparison with natural occurrences
An additional line of comparison is provided by the consideration of natural assemblages and the following transitions: the berthierine-to-chlorite conversion (Iijima and Matsumoto, 1982; Hillier and Velde, 1992), the di- to trioctahedral smectite transition (Inoue, 1987; Inoue and Utada, 1991), and the smectite-to-chlorite conversion (Bettison-Varga and Mackinnon, 1997; Robinson et al., 2002; Schiffman and Staudigel, 1995). The temperature range of stability of chlorite is rather large, between 110ºC and 420ºC (Iijima and Matsumoto, 1982;
Cathelineau and Izquierdo, 1988; Hillier and Velde, 1992; Buatier et al., 1993; Aagaard et al., 2000) indicating a potential formation of chlorite minerals at temperatures close to those of interest in waste disposal.
Figure 7. Comparison of the crystal chemistry of clay particles obtained from the 12-month experiments carried out at 80ºC (open circles), 150ºC (gray diamonds), and 300ºC (gray squares) in the presence of an Fe/C ratio of 0.5 and an L/C ratio of 5.
The coexistence of dioctahedral smectite and chlorite is frequent above 160ºC in geothermal systems where minerals are directly precipitated from a fluid and do not derive from the alteration of a clay precursor (Cathelineau and Nieva, 1985; Cathelineau and Izquierdo, 1988; Bailey, 1994). Fe-rich chlorites have also been described in sandstone reservoirs at tempera- tures of 100 120ºC. Berthierine has been observed mostly in sedimentary basins as an early, low-temperature diagenetic phase (15 40ºC), of which the upper limit of stability is within the 65 130ºC temperature range (Iijima and Matsumoto, 1982; Aagardet al., 2000). Above 160ºC, berthierine is entirely converted to chamosite in natural systems through mixed layering (Hillier and Velde, 1992), and, experimentally, above 200 250ºC (Aagard et al., 2000). Ca-zeolites (laumontite, clinoptilolite, mordenite) are commonly observed associated with smectite and chlorite as alteration products of basic glasses of magmatic rocks submitted to hydrothermal alteration (Cathelineau and Nieva, 1985; Alt et al., 1986;
Schiffman and Fridleifsson, 1991, among others). For diagenetic settings, Gu¨ndogduet al.(1996) suggested that the formation of zeolites and the amount of zeolites being formed could also be controlled by the amount of Fe released during the dissolution of volcanic glass. Those authors showed that a large Fe content favors the formation of smectite rather than zeolite (especially clinoptilolite). The experiments carried out at various Fe/C ratios (Table 1) confirm this suggestion. At 300ºC, zeolites are only observed for experiments with small Fe contents (Fe/C = 0.1) (Guillaume, 2002; Guillaumeet al., 2004; Charpentier et al., 2006). In lower-temperature experiments (80ºC or 150ºC), zeolites do not form probably because of kinetic effects (Guillaume, 2002;
Guillaume et al., 2004; Perronnet, 2004; Charpentieret al., 2006, Perronnet et al., 2008). At the same low temperatures but in experiments with large Fe contents (at the contact with the Fe plate and for Fe/C ratios > 0.5), zeolites were not observed.
The experimental mineral assemblages observed in the present experiments and in experiments described in the literature are, therefore, compatible with those found in nature as the upper boundary of berthierine is at
~120 130ºC and the lowest temperature estimated for chlorite is ~110ºC. Authigenic dioctahedral smectite and chlorite are found in the same samples as co-existing phases, possibly at equilibrium, among a large range of temperature from 160 to 240ºC, while saponites are mostly found in the presence of fluids having a greater ionic strength (close to or above sea water) than hydrothermal waters from geothermal systems related to volcanic activity.
Redox conditions and role of intermediate fO2
The overall consideration of the experimental sets of data indicates that the redox state of the system and the availability of Fe seem to be the two main critical
parameters which control the formation of the Fe-rich mineral phases and their relative abundance. In fact, the main changes in physical-chemical parameters concern the redox conditions which dramatically reduce toward thefO2close to water dissociation, and the production of H2, which has been confirmed experimentally in some Japanese experiments which show the bubbling of H2at the Fe/C interface (Idemitsu et al., 2002). The sub- sequent increase in pH due to Fe dissolution is balanced by smectite-to-berthierine or chlorite conversion.
The above-mentioned sequences are distinct from that proposed for greater fO2, lower temperatures (<150ºC), and very large L/C ratios where the pre- dominant newly formed phases are serpentine-group minerals (odinite cronstedtite, Lantenois, 2003;
Lantenois et al., 2005; Perronnet, 2004; Perronnet et al., 2008). The discrepancy is mostly linked to the role of: (1) the L/C ratio: when <5, the fluid chemistry is buffered by the smectite and its evolution toward 7 A˚ phases seems either impossible or very slow; (2) the redox conditions: under intermediate fO2, Fe2+/Fe3+
phases of the serpentine group are favored, while under the conditions of water instability, Fe-rich dioctahedral or trioctahedral smectite forms at low temperatures (<150ºC), and chlorite or vermiculite form at temperatures above 200 250ºC. A schematic impression of mineral zoning taking into account the redox state, the availability of Fe, and the L/C ratio at low temperatures (<150ºC) is presented in Figure 8.
The nature of the non-swelling Fe-rich clay (7 A˚ vs.
14A˚ phases) is debatable mostly because of the difficulty in determining the exact structure of a phase which is limited in quantity, and mixed mechanically or structurally (mixed-layering) with other minerals such as di- or trioctahedral smectites. A considerable set of arguments exists in favor of the formation of chlorite at high temperature and this mineral is also predicted by geochem- ical modeling of the smectite-Fe interaction, even at 80ºC (Kluska et al., 2002; Monteset al., 2005). Alternately, a series of smectite-Fe experiments resulted in the formation of a 7 A˚ phase at lower temperatures (80ºC, 150ºC) when the Fe/C and L/C ratios are large. Their stability field seems to have been increased by more oxidizing redox conditions, which favor serpentine-group minerals. The two groups of minerals, chlorite and Fe-serpentine, can thus be formed, but the exact time-temperature pairs delimiting their formation remains poorly known. New data presented here show that for 1 y experiments, the temperature boundary could be between 150 and 300ºC, but these data cannot be extrapolated to other time-temperature pairs close to those of interest for waste disposal (e.g. 1000 to 10,000 y/
50 80ºC for instance).
Role of the smectite-Fe contact on the smectite conversion rate
A role of the direct contact between Fe metal and clay has been put forward by Perronnet (2004) as a necessary
288 Mosser-Ruck et al. Clays and Clay Minerals
condition for the smectite–berthierine conversion. The results of the present study show, however, that 7 A˚ or 14A˚ phases may form at distinct distances from the Fe plate, but with distinct Fe contents. The clay minerals crystallized at more than a few mm from the Fe plate are Mg-rich whatever their nature (chlorite or vermiculite).
In any case, the rate of alteration of the clay barrier will be extremely dependent on the ability of Fe to migrate through the clay barrier as Fe2+. This process is controlled by: (1) the rate of corrosion of iron; (2) the rate of formation of a possible crust of magnetite (or siderite whenPCO2is increased) at the interface with the Fe canister, which may reduce the amount of Fe2+
released to the system; and (3) the dynamics of migration of Fe through diffusion processes.
Consequences for swelling properties
The transformation of montmorillonite into the association of serpentine-like or chlorite phases might lead to significant modifications of the material proper- ties (Devineau et al., 2005). On the contrary, the formation of saponite or vermiculite, e.g. minerals having swelling properties has a less critical effect on the swelling properties of the bentonite, than the formation of chlorite or a serpentine-like phase. The vermiculitization of montmorillonite is accompanied by a reduction of the structural Fe in the newly formed clay particles. The reduction in Fe may influence the properties of smectite (Stucki and Tessier, 1991;
Stucki et al., 1984a, 1984b; Lear and Stucki, 1989).
Lear and Stucki (1989) described, for example, the decrease in swelling capacity and of the specific surfaces of nontronite due to the collapse of some interlayers
after Fe reduction. In addition, the proportion of swelling phases decreases as some of the material is converted to a non-swelling phase (zeolite). Besides, zeolites such as mordenite have a large cation exchange capacity and large adsorption capacity (Pabalan, 1994).
The exchange and adsorption capacities of the zeolite + vermiculite assemblage may be thus as important as those of the initial bentonite.
Extrapolation of results to small-water-content bentonite and low temperatures
With regard to a nuclear waste repository, the experi- mental simulations above have demonstrated an evolution toward clay minerals with smaller CEC values. The transformation was observed to its greatest extent nearest to the contact with the added metallic Fe plate and under specific experimental conditions favoring the reactions.
Prediction of the potential behavior of smectite in the presence of a steel container and their related alteration products, under nuclear-waste-repository conditions, is made difficult by the likely temperature range of 25 90ºC. Under such low temperatures, very little miner- alogical change could be determined from experiments carried out over short durations (<1 2 y) and for liquid/
mineral ratios typical of an engineered barrier (<0.1).
Experiments carried out at higher temperatures could, however, be extrapolated to lower temperatures as the type of mineralogical changes observed at high temperature is predicted by numerical modeling either at high (300ºC) or much lower temperatures (80 100ºC). However, kinetic predictions using constraints from temperature-time pairs still require experimental calibration.
ACKNOWLEDGMENTS
The experiments were conducted at the Ge´ologie et Gestion des Ressources Mine´rales et Energe´tiqueslabora- tory (G2R, Nancy-Universite´, CNRS, CREGU, Van- doeuvre-le`s-Nancy, France). The XRD and FTIR spectroscopy data were collected at the Laboratoire Environnement et Mine´ralurgie (LEM, CNRS-INPL, Van- doeuvre-le`s-Nancy, France), and the EDS and HRTEM data at Nancy Universite´ (Vandoeuvre-le`s-Nancy, France).
The authors thank J. Ghanbaja (UHP, Vandoeuvre-le`s- Nancy, France) for the EDS analyses, and K. Devineau and I. Bihannic (LEM, Vandoeuvre-le`s-Nancy, France) for their help and technical assistance with the XRD. This research was financially supported by ANDRA - Agence Nationale pour la gestion des De´chets RadioActifs (French national agency for the management of radioactive waste).
REFERENCES
Aagaard, P., Jahren, J.S., Harstad, A.O., Nilsen, O., and Ramm, M.(2000) Formation of grain-coating chlorite in sandstones. Laboratory synthesizedvs. natural occurrences.
Clay Minerals,35, 261 269.
Alt, J.C., Honnorez, J., Laverne, C., and Emmermann, R.
(1986) Hydrothermal alteration of 1 km section through the upper oceanic crust, DSDP Hole 504B: Mineralogy, chemistry and evolution of seawater-basalt interaction.
Journal of Geophysical Research,91, 10309 10335.
Figure 8. Schematic impression of mineral zoning taking into account the redox state, the availability of Fe and the L/C ratio at low temperature (<150ºC).
Bailey, S.W. (editor) (1994)Hydrous Phyllosilicates. Reviews in Mineralogy, 19, Mineralogical Society of America, Washington, D.C., 725 pp.
Bettison-Varga, L. and Mackinnon, I.D.R. (1997) The role of randomly mixed-layered chlorite/smectite in the transforma- tion of smectite to chlorite. Clays and Clay Minerals, 45, 506 516.
Bildstein, O., Trotignon, L., Perronnet, M., and Jullien, M.
(2006) Modelling iron-clay interactions in deep geological disposal conditions.Physics and Chemistry of the Earth,31, 618 625.
Buatier, M., Ouyang, K., and Sanchez, J.P. (1993) Iron in hydrothermal clays from the Galapagos spreading centre mounds: consequences for the clay transition mechanism.
Clay Minerals,28, 64 1 655.
Cathelineau, M. and Izquierdo, G. (1988) Temperature- composition relationships of authigenic micaceous minerals in the Los Azufres geothermal system. Contributions to Mineralogy and Petrology,100, 4 18 428.
Cathelineau, M. and Nieva, D. (1985) A chlorite solid solution geothermometer. The Los Azufres (Mexico) geothermal system. Contributions to Mineralogy and Petrology, 91, 235 244.
Cathelineau, M., Mosser-Ruck, R., and Charpentier, D. (2001) Interactions fluides/argilites en conditions de stockage profond des de´chets nucle´aires. Inte´reˆt du couplage expe´ri- mentation/mode´lisation dans la compre´hension des me´ca- nismes de transformation des argiles et la pre´diction a long t er m e d u co mp o r t em e n t d e l a b a r r i e` r e a r gi le us e . Pp. 305 341 in:Actes des Journe´es Scientifiques ANDRA.
EDP Sciences, Nancy, France.
Charpentier, D., Devineau, K., Mosser-Ruck, R., Cathelineau, M., and Villie´ras, F. (2006) Bentonite-iron interactions under alkaline condition: an experimental approach.Applied Clay Science,32, 1 13.
Chermak, J.A. (1992) Low temperature experimental investi- gation of the effect of high pH NaOH solutions on the opalinus shale, Switzerland. Clays and Clay Minerals, 40, 650 658.
Devineau, K., Charpentier, D., Villie´ras, F., Mosser-Ruck, R., Maddi, S., Barres, O., and Razafitianamaharavo, A. (2005) MX80/iron interactions at 80, 150, and 300ºC under alkaline conditions: influence of the mineralogical transformations on texture.Proceedings of the International Meeting: Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Tours, France, 339 pp.
Garrels, R.M. and Christ, J.C. (1965) Solutions, minerals, and equilibria. Freeman, Cooper, San Francisco, California, USA.
Grauby, O., Petit, S., Decarreau, A., and Baronnet, A. (1993) The beidellite-saponite solid-solution: An experimental approach.European Journal of Mineralogy,5, 623 635.
Guillaume, D. (2002) Etude expe´rimentale du syste`me fer- smectite en pre´sence de solution a` 80ºC et 300ºC. PhD Thesis, Universite´ Henri Poincare´, Nancy I, Nancy, France, 208 pp.
Guillaume, D., Neaman, A., Cathelineau, M., Mosser-Ruck, R., Peiffert, C., Abdelmoula, M., Dubessy, J., Villieras, F., Baronnet, A., and Michau, N. (2003) Experimental synthesis of chlorite from smectite at 300ºC in the presence of metallic Fe.Clay Minerals,38, 281 302.
Guillaume, D., Neaman, A., Cathelineau, M., Mosser-Ruck, R., Peiffert, C., Abdelmoula, M., Dubessy, J., Villie´ras, F., and Michau, N. (2004) Experimental study of the transformation of smectite at 80ºC and 300ºC in the presence of Fe oxides.
Clay Minerals,39, 17 34.
Gu¨ndogdu, M.N., Yalc¸in, H., Temel, A., and Clauer, N. (1996) Geological, mineralogical and geochemical characteristics of zeolite deposits associated with borates in the Bigadic¸,
Emet and Kirka Neogene lacustrine basins, Western Turkey.
Mineralium Deposita,31, 4 92 513.
Hillier, S. and Velde, B. (1992) Chlorite interstratified with a 7 A˚ mineral: an example from offshore Norway and possible implications for the interpretation of the composition of diagenetic chlorites.Clay Minerals,27, 4 75 486.
Idemitsu, K., Yano, S., Xiaobin, X., Inagaki, Y., and Arima, T.
(2002) Diffusion behaviour of iron corrosion products in buffer materials.Materials Research Society Symposium Proceedings,713, 113 120
Iijima, A. and Matsumoto, R. (1982) Berthierine and chamosite in coal measures of Japan. Clays and Clay Minerals, 30, 264 274.
Inoue, A. (1987) Conversion of smectite to chlorite by hydrothermal diagenetic alterations, Hokuroku Kuroko mineralization area, northeast Japan. Pp. 158 164in:
Proceedings of the International Clay Conference, Denver, 1985 (L.G. Schultz, H. van Olphen, and F.A. Mumpton, editors). The Clay Minerals Society, Bloomington, Indiana, USA.
Inoue, A. and Utada M. (1991) Smectite-to-chlorite transfor- mation in thermally metamorphosed volcanoclastic rocks in the Kamikita Area, north Honshu, Japan. American Mineralogist,76, 628 640.
Kluska, J.M., Fritz, B., and Clement, A. (2002) Predictions of the mineralogical transformations in a bentonite barrier surrounding an iron radioactive container. International Meeting, Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, December 9–12, 2002, Reims, France: O-10b-5, pp. 153 154.
Lantenois, S. (2003) Re´activite´ fer me´tal/smectites en milieu hydrate´ a` 80ºC. PhD Thesis, Universite´ d’Orle´ans, Orle´ans, France, 188 pp.
Lantenois, S., Lanson, B., Muller, F., Bauer, A., Jullien, M., and Planc¸on, A. (2005) Experimental study of smectite interaction with metal Fe at low temperature: 1 Smectite destabilization.Clays and Clay Minerals,53, 597 612.
Lear, P.R. and Stucki, J. W. (1989) Effects of iron oxidation state on the specific surface area of nontronite.Clays and Clay Minerals,37, 54 7 552.
Montes, H.G., Fritz, B. Clement, A., and Michau, N. (2005) Modelling of transport and reaction in an engineered barrier for radioactive waste confinement. Applied Clay Science, 29, 155 171.
Pabalan, R.T. (1994) Thermodynamics of ion exchange between clinoptilolite and aqueous solutions of Na+/K+ and Na+/Ca2+.Geochimica et Cosmochimica Acta,58(21), 4573 4590.
Perronnet, M. (2004) Re´activite´ des mate´riaux argileux dans un contexte de corrosion me´tallique. Application au stock- age des de´chets radioactifs en site argileux. PhD Thesis, Institut National Polytechnique de Lorraine, France, 288 pp.
Perronnet, M., Jullien, M., Villie´ras, F., Raynal, J., Bonnin, D., and Bruno, G. (2008) Evidence of a critical content in Fe(0) on Foca7 bentonite reactivity at 80ºC.Applied Clay Science, 38, 187 202.
Robinson, D., Schmidt, S.Th., and Santana de Zamora, A.
(2002) Reaction pathways and reaction progress for the smectite-to-chlorite transformation: evidence from hydro- thermally altered metabasites. Journal of Metamorphic Geology,20, 167 174.
Saumagne, P. and Josien, M.L. (1962)Advances in Molecular Spectroscopy. Pergamon Press. London, Vol. II, p. 1033.
Savage, D., Watson, C., Benbow, S., and Wilson, J. (2010) Modelling iron-bentonite interactions. Applied Clay Science,47, 91 98.
Schiffman, P. and Fridleifsson, G.O. (1991) The smectite- chlorite transition in drillhole Nj-15, Nesjavellir geothermal field, Iceland: XRD, BSE, and Electron Microprobe
290 Mosser-Ruck et al. Clays and Clay Minerals
Investigations. Journal of Metamorphic Geology, 9, 679 696.
Schiffman, P. and Staudigel, H. (1995) The smectite to chlorite transition in a fossil seamount hydrothermal system: the basement complex of La Palma, Canary Islands.Journal of Metamorphic Geology,13, 4 87 498.
Schlegel, M.L., Bataillon, C., Benhamida, K., Blanc, C., Menut, D., and Lacour, J.L. (2008) Metal corrosion and argillite transformation at the water-saturated, high-tem- perature iron-clay interface: A microscopic-scale study.
Applied Geochemistry,23, 2619 2633
Stucki, J.W. and Tessier, D. (1991) Effects of iron oxidation state on the texture and structural order of Na-nontronite.
Clays and Clay Minerals,39, 137 143.
Stucki, J.W., Golden, D.C., and Roth, C.B. (1984a) Effects of reduction and reoxidation of structural iron on the surface charge and dissolution of dioctahedral smectites.Clays and Clay Minerals,32, 350 356.
Stucki, J.W., Low, P.F., Roth, C.B., and Golden, D.C. (1984b) Effects of oxidation state of octahedral iron on clay
swelling.Clays and Clay Minerals,32, 357 362.
Toppani, A., Robert, F., Libourel, G., de Donato, P., Barres, O., d’Hendecourt, L., and Ghanbaja, J. (2005) A ‘dry’
condensation origin for circumstellar carbonates. Nature, 437, 1121 1124.
Wilson, J., Ragnarsdottir, K.V., Savage, D., Cuadros, J., Cressey, G., Cressey, B., and Shibata M. (2005) The effect of iron on bentonite stability: an investigation into reactions between native Fe, magnetite, montmorillonite and aqueous solutions. Proceedings of the International Meeting, Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Tours, France, p. 39.
W i l s o n, J . , C r e s s e y , G . , C r e s s e y , B . , C u a d r o s , J . , Ragnarsdottir, K.V., Savage, D., and Shibata M. (2006) The effect of iron on montmorillonite stability. (II) Experimental investigation. Geochimica et Cosmochimica Acta,70, 323 336.
(Received 6 April 2009; revised 21 January 2010; Ms. 303;
A.E. P. Komadel)
